xir PHYLUM CHORDATA 407 



of a number of membrane bones ; the skull is autostylic, the 

 lower jaw articulating with a palato-quadrate process (pal}, 

 corresponding to the palato-quadrate of the dogfish, but im- 

 movably fixed to the side of the skull. There are four or 

 five cartilaginous branchial arches (br). The gills are cov- 

 ered over by an operculum. A cloaca is present, and the 

 intestine contains a spiral valve. The structure of the heart 

 is more complicated than in ordinary fishes, owing to the 

 sinus venosus and the auricle being both imperfectly divided 

 into two parts. There is a contractile conus arteriosus, which 

 has a spirally twisted form, and is partly or completely divided 

 internally by a longitudinal septum. A pulmonary artery, 

 carrying blood to the lung or lungs, is given off from the 

 afferent branchial system. The blood is returned from 

 the lung or lungs by means of a pulmonary vein, which 

 opens into the left division of the sinus venosus, and 

 through it reaches the left division of the auricle. 



CLASS III. AMPHIBIA 



The larvae of the newts or efts, and those of the frogs 

 (familiar to every one under the name of tadpoles), bear a 

 close resemblance in form and movements to the fishes. 

 But, as they approach maturity, these fish-like larvae undergo 

 a metamorphosis, fitting them for the amphibious mode of 

 life which gives its name to the class : pentadactyle limbs are 

 developed, and lungs for breathing air take the place of gills 

 as the organs of respiration, while a corresponding advance to 

 a higher type of structure occurs in the other parts. The 

 Amphibia thus occupy an intermediate position between the 

 fishes on the one hand and the higher air-breathing Verte- 

 brates (reptiles, birds, and mammals) on the other. In 

 addition to the possession of limbs constructed on the pen- 



